Since he won an upset US Senate special election victory in Massachusetts in 2010, Brown’s military service has been an essential part of his political profile. But he has said it is much more than that, calling it a meaningful part of the sweep of his career.

Brown, who joined the Massachusetts Army National Guard in 1979, transferred to the Maryland National Guard in 2012. A lawyer, he has been serving both at the Pentagon and at the nearby National Guard Bureau headquarters.

The former senator is set make his New Hampshire bid official at a campaign rally in Portsmouth on Thursday evening, according to an e-mail he sent Monday to supporters.

The long-expected move comes as Brown has been ramping up campaign activity in his nascent effort to unseat Democrat Jeanne Shaheen. On Monday evening, he held his first fund-raiser. Top Senate Republicans, including Minority Leader Mitch McConnell of Kentucky and John Cornyn of Texas attended the event, held at the Dubliner, a wood-paneled Irish pub, blocks from the US Capitol.

While Brown mingled with attendees in a private room, a small number of protesters held signs, one of which depicted the crossed-out phrase “Scott Brown 4 Massachusetts.”

The invitation to the event called for donations from $250 to $1,000 for individuals and from $1,000 to $2,500 for political action committees.

Brown, who was unseated by Elizabeth Warren in 2012, was among the Republican Party’s most prolific Senate fund-raisers during his last run. During the 2011-2012 election cycle, he raised more than $28 million, according to Federal Election Commission filings.

His fund-raising prowess is one of the reasons national Republicans believe he has a strong shot of unseating Shaheen, a former New Hampshire governor who was first elected to the US Senate in 2008. Democrats are bullish that Shaheen, who had $3.4 million in the bank at the end of 2013 and is well known in the state, will win reelection in November.

But specialists in campaign finance law said there is little difference between that exploratory phase and a full-fledged campaign.

A candidate who has filed a “statement of candidacy,” as Brown did in March, is able to raise money just as an official candidate could, the specialists said.

Paul S. Ryan, senior counsel with the Campaign Legal Center, said “once you file that paperwork with the FEC, you’re a candidate in the eyes of the FEC.”

Warren had a message for Brown on Monday: He should be prepared for a hard fight against Shaheen. “Jeanne is tough, she is independent, and she is strong,” Warren said after attending a City Hall press conference in Boston.

“This will be up to the people of the Commonwealth . . . whoops, this will be up to the people of New Hampshire,” Warren said, laughing. “But I think Scott Brown is going to have his hands full.”

In his e-mail to supporters announcing his Thursday rally, Brown emphasized his ties to the state where he spent his early childhood.

“Portsmouth played a special role in my childhood. I remember our house on Islington Street, strolling through Strawbery Banke with my grandfather, and catching a show at Prescott Park,” he wrote.

“Now, Portsmouth is going to be the start of the next chapter in my life,” he added, inviting supporters to join him at his kickoff event.

Before facing Shaheen, Brown will first have to win a GOP primary. Other Republican candidates include former US senator Bob Smith, former state senator Jim Rubens, and conservative activist Karen Testerman.

Jim O’Sullivan, Noah Bierman and Bryan Bender of the Globe staff contributed to this report. Joshua Miller can be reached at joshua.miller@globe.com. Follow him on Twitter @jm_bos.